This invention relates to a vehicular power supply system, and more particularly to a power supply system of the kind described above which is suitable for supplying two different d.c. voltages.
In a cold district, it is especially difficult to start an engine of a vehicle such as a car or a truck which runs outdoors in the winter season. Such a difficulty is attributable to an increased viscosity of engine oil, degradation of the ignitability of fuel, etc.
A power supply system which obviates such a difficulty is disclosed in, for example, U.S. Pat. No. 4,045,718. According to the disclosure of this United States patent, two 12-volt batteries commonly used as a power source are connected in series to provide a power source of 24 volts, and this 24-volt power source is used to drive a starting motor for starting an engine, thereby enhancing the starting efficiency of the motor so that the engine can be made easy to start.
In this prior art, the voltage regulator is provided only for 12-volt output to maintain thereof, and no voltage regulation is made on another voltage output of the generator.
Therefore, the voltage changed in the second battery tends to change depending on the value of current flowing through the second set of armature windings, that is, depending on the relative magnitude of 24-volt output current. Thus, although a self-current limiting function similar to that of a conventional automatic generator works, the diameter of the conductors of the second set of armature windings cannot be decreased beyond a limit from the aspect of the resistance against vibration, and, for the above reason, an output having an excessive margin relative to the rated output tends to appear from the second set of armature windings. Consequently, such a problem has arisen frequently in which flow of an excessively large current through the high-voltage (24 volts) supply side of the system results in objectionable thermal breakdown of the elements including the associated rectifier. Further, there has been the tendency that the second battery is overcharged or undercharged.